Fan for a turbomachine

ABSTRACT

The invention proposes a fan, in particular for a turbomachine of small size such as a jet engine, having a hub ratio which corresponds to the ratio of the diameter of the inner limit of the incoming air stream ( 26 ) at the radially inner ends of the leading edges of the fan blades ( 10 ), divided by the diameter of the circle around which the outer ends of the fan blades pass, having a value of between 0.20 and 0.265.

The present invention concerns obtaining specific dimensions for a fan,particularly for a turbomachine, such as a jet engine.

The invention constitutes a genuine technical challenge and isspecifically of value when it relates to turbomachines, the externaldimensions of which have been designed to adapt to the field of businessaviation. Typically, these turbomachines, relatively small in size, havean intake diameter, defined by the upstream diameter of the turbomachinestream, of between 900 mm and 1550 mm, in order to possess dimensionsclosely related to a total mass and suitable for installation onaircraft of the business jet type.

As on any type of turbomachine, the developments concerning this type ofsmall-sized turbomachine mainly concern improvement in performances, areduction in consumption and a saving in weight. There are many lines ofdevelopment in this respect, which may for example relate to the choicesof materials, study of the shapes of the blades, optimisation of themechanical links among the components, prevention of leaks, etc.

One of the lines of development generally pursued involves reducing thehub ratio of the turbomachine fan. This hub ratio is the ratio betweenthe external diameter of the hub at the leading edges of the fan bladesand the diameter of the circle around which the radial ends of these fanblades pass. The reduction in the hub ratio generally means a radialreduction in the size of the hub and therefore a saving in weight, butalso involves an increase in the suction section of the turbomachine,incurring an increase in the air flow propelling the turbomachine andtherefore enhanced performances. However, taking into account thecurrent know-how in the design and manufacture of small-sizedturbomachines, such as those having an intake diameter defined above,this type of turbomachine is considered as not allowing a reduction inthe external diameter of the hub, particularly at the leading edges ofthe fan blades, to below the diameter currently employed which istypically between 570 and 585 mm. Indeed, the current dimensions of themechanical elements forming the hub are considered non-reducible, mainlyfor obvious reasons of radial mechanical strength of the blades,torsional resistance, tolerances and methods of manufacture andaccessibility to tools, etc.

In contradiction to these technical prejudices, the invention proposes achoice of specific dimensions for a turbomachine fan affording asignificant improvement in performances and weight.

To this end, the invention proposes a fan, particularly for aturbomachine such as a jet engine, wherein the fan comprises at theintake, fan blades, an annular casing, a hub rotating around an axis ofthe turbomachine and bearing the blades, which extend radially inrelation to said axis in an annular stream delimited internally by thehub and externally by the annular casing, wherein said fan has an intakediameter, which corresponds to the diameter of the circle comprising theradially outer ends of the blades, of a value of between 900 mm and 1550mm and possesses a hub ratio, which corresponds to the ratio of thediameter of the inner limit of the stream at the radially inner ends ofthe leading edges of the fan blades, divided by the intake diameter, ofa value of between 0.20 and 0.265.

According to a first embodiment, the hub comprises a fan discconstructed in one piece with the blades.

According to a second embodiment, the hub comprises a fan disccomprising at its outer circumference substantially axial ribs formed inalternation with grooves in which the roots of the blades are engaged.

More specifically, an intake diameter of between 900 mm and 1200 mm isproposed, in order to obtain even more advantageous results in terms ofweight. As will be explained later, the specific choice of such anexternal diameter is all the more subject to technical prejudice.

Furthermore, a specific mechanical arrangement of the rotor of this fanis proposed that is particularly well suited to this choice ofdimensions.

Usually, the rotor of a turbomachine fan comprises a disc constructed inone piece with the blades, or bearing at its outer circumference blades,the roots of which are engaged in substantially axial grooves of theouter circumference of the disc.

In a case in which the blades are engaged on the disc, the blades areheld radially on the disc by positive interlocking of their roots withthe grooves of the disc, wherein the blade roots are for example of thedovetail type. Inter-blade platforms are mounted on the disc between thefan blades. The disc is usually equipped with balancing systems (knownas “leeks”) extending radially inwards.

In the current technology, the blades are axially maintained on the discby means that are mounted on the disc, upstream and downstream from theblades, which prevent the blade roots from moving axially in the groovesof the disc.

The retaining means located downstream from the blades comprise forexample at least one blade root hook which is engaged in a notchmachined on an upstream end portion of the low-pressure compressorarranged downstream from the fan. In order to allow fixing of thesehooks in the notches of the low-pressure compressor, the disc groovesneed to be expanded radially in relation to the blade roots. It is thuspossible to move the blades axially in the bottom of the grooves andposition the blade root hooks aligned radially opposite the notches. Itis subsequently possible to raise the blades radially in the groovesusing adequately thick shims, arranged at the bottom of the grooves, inorder to engage the blade root hooks in the notches and keep the bladesin the top position.

The retaining means located upstream comprise for example an annularflange attached and fastened to the upstream end of the disc. The flangeis mounted coaxially on the disc and comprises a scalloped sectioninteracting with a corresponding scalloped section of the disc. Thisflange secures the ring axially on the disc and is secured againstrotation in relation to the disc. The outer circumference of the flangeis axially supported on the blade roots for their axial retention in thedownstream direction, whilst its inner circumference is applied andfixed to a corresponding annular collar of the disc. The outercircumference of the flange furthermore comprises fastening pins for theupstream ends of the inter-blade platforms.

A substantially truncated cone-shaped shell mounted on the disc,upstream from the blades, delimits internally the annular incoming airstream into the turbomachine. This shell comprises near its downstreamend a radially inner annular collar applied axially to theaforementioned flange and which is fixed with the flange to the collarof the disc by bolts.

A truncated cone-shaped cowling is furthermore mounted on theaforementioned shell, on the upstream portion of the latter, by means ofother bolts, engaged in holes of the collars of the cowling and of theshell and which are located radially inside the bolts for fixing theshell to the disc.

Whether a disc constructed in one piece with the blades is involved, ora disc comprising grooves in which the blades are engaged, the disc isfixed to a downstream drive shaft via a radial annular collar of thedisc fixed to a radial annular collar of the shaft by means of a seriesof nuts aligned circumferentially and screwed axially through thecollars.

In order to perform assembly and disassembly of the fan rotor, it isnecessary to be able to gain access to these nuts axially using a tool.For this purpose, the operator must have sufficient space availablearound the central axis. If the fan diameter is small and in particularif the hub ratio of the fan is that mentioned in this patentapplication, the prior art structure described above does not allowaccess to the aforementioned nuts. Indeed, the balancing systems(“leeks”) of the disc are in this case formed in the axial alignment ofthe nuts and markedly reduce the available space around the central axisupstream from the drive shaft for access to the nuts.

Furthermore, the loads transmitted by the shaft to the disc are borneentirely by the bolted aforementioned collars, which are elementsparticularly sensitive to deformations and breaks in the torquetransmission chain from the shaft to the fan disc. In the case mentionedabove, since the radial and circumferential dimensions of these collarsare very small, there are major risks of deformation and breakage of thelatter during operation.

Whether a disc constructed in one piece with the blades is involved, ora disc comprising grooves in which the blades are engaged, the prior arttherefore does not allow, in accordance with the technical prejudicementioned, formation of a fan of a dimension and hub ratio defined bythe invention.

Document EP 1 357 254 also discloses a fan rotor, the structure of whichhas large radial and axial design envelopes.

Providing a simple, effective and economical solution to this problem isan objective sought here, also as an aim in itself, possibly regardlessof the constraints of intake diameter and hub ratio mentioned above andclaimed.

To this end, it is proposed that provision be made for a torquetransmission between the fan disc and a downstream drive shaft centredon the same axis, said torque transmission being achieved via an annularrow of axial splines of the disc that interact with an annular row ofaxial splines of the shaft.

Preferentially, the splines of the disc are formed on the inside surfaceof a cylindrical wall of the disc, wherein said cylindrical wallsurrounds the drive shaft.

According to another characteristic, the cylindrical surface is formedat the downstream end of the disc and is connected to the remainder ofthe disc via a truncated cone-shaped wall flaring out in the upstreamdirection.

Advantageously, at least one annular shoulder is formed on the surfaceof the drive shaft and is in axial abutment downstream against a stop onthe disc.

The stops may be formed by the downstream end of the cylindrical walland/or a radial annular edge extending inside the truncated cone-shapedwall.

Preferentially, a nut is installed on a thread of the outer surface ofthe upstream end of the shaft and forms an axial abutment from theupstream direction on at least one stop of the disc, in order tomaintain the stop clamped axially between said nut and a shoulder of theshaft.

The nut typically has a diameter of between 105 mm and

135 mm and preferably between 115 mm and 125 mm.

The structure defined above offers a more resistant method of torquetransmission than that involving bolted radial collars. Indeed, when acollar connection involves the presence of radial walls that arerelatively weak during bending and the presence of bolts inserted into alimited number of orifices where the loads are concentrated along thecircumference of the collars, the spline connection allows distributionof the torque over the entire circumference of splined cylindrical wallscapable of better withstanding high shear loads.

Whether a disc constructed in one piece with the blades is involved, ora disc comprising grooves in which the blades are engaged, the structuredefined above therefore solves the problem of mechanical strengthbetween the transmission shaft and the disc within the context of thefan of a dimension and hub ratio defined by the invention.

The fan rotor described above, the proposed design of which is directlyrelated to the choice of the hub ratio made within the context ofexecuting a small-sized turbomachine, was furthermore developed inconnection with the technical environment described below. Thisenvironment proposed by the inventors allows in particular formation ofa particular arrangement of the fan rotor that provides a solution formounting the disc on the drive shaft in order to obtain the splinedconnection expressed in this patent application.

The specific choice of hub ratio mentioned in this patent applicationinvolves in fact an overall reduction in the dimensions of the fan discof the turbomachine in relation to the prior art. This disc has anexternal diameter, the value of which is in this case typically includedbetween 245 and 275 mm. If the blades are engaged on the disc, itremains necessary however that this disc should meet the constraintsrelating to keeping the fan blades in operation, the number anddimensions of which remain relatively the same in relation to the priorart. To this end, the number of blades is preferably between 17 and 21blades and more specifically between 18 and 20 blades. The height andwidth of the grooves of the disc must, furthermore, according to theknowledge of the current technology, not undergo any reduction indimensions in order on the one hand to allow engagement of thedownstream hooks, for axial retention of the blades, mentioned in thisapplication and on the other hand be adapted to the size of the bladeroots, the dimensions of which have not been reduced in order to bearthe rotating blades.

The simultaneous requirements of preservation of the dimensions of thegrooves of the disc and reduction in the overall diameter of the discinvolve in this case necessarily a decrease in the width, i.e. in thecircumferential dimension of the ribs of the disc. The ribs of the fandisc, finer in this case than in the prior art proposing a higher hubratio, consequently display greater weakness and a higher risk ofbreakage in relation to the torque sustained during operation than theribs of the prior art.

In order to solve this problem, it was proposed to construct the fandisc in an inconel alloy, which is very strong. This alloy is howeververy heavy, which harms the overall performances of the turbomachine andtherefore does not represent a satisfactory solution.

Within the context of the fan rotor described above, it was noticed,unexpectedly, that when the blades are engaged on the disc, the axialsecuring of the blades performed by a specific arrangement of the disc,of a retaining flange upstream from the disc, of a ring and of a fancowling, with this assembly having been developed by the inventors, wassufficiently effective and resistant in order to dispense with the axialsecuring performed by the downstream hooks of the fan roots engaged inthe low-pressure compressor, with respect to a turbomachine, thedimensions of which have been specified above.

This specific arrangement of axial securing of the blades consists of anannular cowling mounted on the disc upstream from the blades and meansof axial retention of the blades on the disc comprising a flangeinstalled in an annular recess of the disc and forming an axial abutmentof the blade roots, wherein the flange comprises a scalloped radialannular edge interacting with a scalloped radial annular edge of theannular recess of the disc, so as to ensure annular securing of theflange in the annular recess of the disc and means of preventingrotation of the flange, comprising a ring equipped with ears extendingradially towards the inside and formed with means on fixing on anupstream radial face of the disc, wherein said cover is fixed to thedisc by means of fixing common to the means of fixing of at least someears of the ring on the disc, wherein the ring furthermore comprises atleast one radial protrusion interacting with an additional stop of theflange, so as to prevent rotation of the flange in relation to the ring.

The inventors have therefore advantageously eliminated the downstreamhook for axial securing of the blades and have consequently been able toreduce the radial height of the grooves of the fan disc, one portion ofwhich was previously reserved for installing the downstream hooks, at aheight typically included between 18 and 22 mm.

The reduction in the radial dimension of the grooves directly involves aradial reduction in the ribs that allows formation of the internalsurface of this disc by a balancing profile derived from a truncatedcone-shaped bore coaxial to the axis of the fan and of which the radiusincreases in the upstream to downstream direction. This balancingprofile, in addition to adequately balancing the fan disc, has a minimumdiameter, upstream, of a value typically included between 120 and 140mm, which is larger than the minimum diameter of the balancing profilewith “leeks” used for the higher grooves, for an equivalent externaldiameter of the disc.

This new disc balancing profile provides a larger annular space in themiddle of the fan disc for axial passage of tools required for mountingand clamping the fan disc on the drive shaft of the turbomachine, bymeans of an arrangement making use of a spline connection, as describedin the present patent application.

Furthermore, reduction in the radial dimension of the fan disc groovessubsequently gives the latter more compact proportions betterwithstanding shear torques during operation. By means of the solutionshown here, the structure of the ribs of the fan disc therefore yields astructure that is sufficiently strong in order to be formed of titaniumalloy which is much lighter than an inconel alloy.

It is therefore possible to propose, in case of a fan rotor comprisingblades engaged on a disc, that said fan rotor be devoid of means ofaxial retention of the fan blades on the fan disc, downstream from theblades. This fan rotor solely comprises the upstream flange as describedin the present patent application as means of axial retention of theblades. This specific feature is particularly relevant within thecontext of fans of small-sized turbomachines relating to the inventionand having the dimensions and hub ratio described above. It is thereforeproposed here, for this type of fan, that the fan disc be constructed oftitanium alloy, more specifically an alloy of type TA6V or TA17(TA5CD4).

Furthermore, another further aspect of the present subject concerns, ifthe disc and blades are executed other than in a single piece, the shimswhich are ordinarily used at the bottom of the grooves in order to keepthe blades upright against the ribs. These shims must in this case havethe functions of limiting the displacement of the blade roots in thegrooves during operation, of protecting the bottoms of the grooves andof cushioning the blades in case of their breakage or during ingestionof a large body by the turbomachine. In order to meet these constraintsin the best possible manner, particularly within the new contextdescribed above, specifically within the context of reducing the radialdimension of the grooves mentioned in this patent application, thepresent patent application proposes a shim that has been renderedradially thinner versus the pre-existing solutions and having a radialthickness typically included between 1 and 3 mm, more particularly equalto 2 mm, its being specified that provision could be made for a shim ofthis kind even regardless of the constraints of intake diameter and hubratio mentioned above and claimed. Each shim more specifically adoptsthe form of a two-sided plate, lying along the axis of the fan andcapable of being placed against the bottom of one of the grooves.Preferentially, this shim is symmetrical in the three axial, radial andcircumferential directions, which avoids any installation errors. Eachside of the shim advantageously possesses lateral or circumferentialedges, which are chamfered, wherein each chamfer forms an angle of 10°,plus or minus 2°, with one side. According to another particularity, thechamfers of each radially opposite side join at the lateral ends of theshim, forming the two lateral edges of the shim. The junction anglesbetween the sides of the shim and the chamfers can be softened so as toexhibit a curvature radius of between 1.50 mm and 1.80 mm and morespecifically of between 1.60 mm and 1.70 mm and preferentially equal to1.65 mm. The junction angles between the chamfers forming the lateraledges of the shim can be softened so as to exhibit a curvature radius ofbetween 0.45 mm and 0.75 mm and more specifically of between 0.52 mm and0.68 mm and preferentially equal to 0.6 mm. According to one particularembodiment, each shim has a lateral dimension of between 17.0 mm and18.2 mm and more specifically equal to 17.6 mm.

The various different aspects of the solutions presented here will bebetter understood and other details, characteristics and advantagesthereof will become more clearly apparent in reading the followingdescription, made by way of a non-restrictive example with reference tothe appended drawings in which:

FIG. 1 is a perspective view, partially torn away, of a turbomachineaccording to the prior art,

FIG. 2 is a partial, cross-sectional axial diagrammatic half-view of aturbomachine fan according to the prior art,

FIG. 3 is a partial, cross-sectional axial diagrammatic half-view, trueto scale, of a turbomachine fan according to the invention, in the casein which the blades are engaged in the grooves of the disc,

FIG. 4 is a partial, cross-sectional axial diagrammatic half-view, trueto scale, of a turbomachine fan according to the invention, in the casein which the blades are formed in one piece with the disc,

FIG. 5 is a perspective view of an updated fan rotor with the cowlingtorn away, for the case in FIG. 3,

FIG. 6 is a front view of the same assembly as that in FIG. 5,

FIGS. 7, 8 and 9 are respective view of the cross-sections A-A, B-B andC-C in FIG. 6,

FIG. 10 is a perspective view of a shim used in the fan according to theinvention, for the case in FIG. 3,

FIG. 11 is a cross-sectional view of the same shim. Reference will bemade first of all to FIGS. 1 and 2, which therefore illustrate aturbomachine fan according to the technology prior to the presentinvention.

This fan comprises blades 10, carried by a disc 12, surrounded by anouter annular casing 8, between which (blades) inter-blade platforms(not illustrated) are interposed, wherein the disc 12 is fixed to theupstream end of a turbomachine shaft 13.

Each fan blade 10 comprises a vane 16 connected at its radially innerend to a root 18 which is engaged in a substantially axial groove 20 ofa shape matching the disc 12, formed between two ribs 22 of the disc 12and allowing radial retention of this blade 10 on the disc 12. A shim 24is interposed between the root 18 of each blade 10 and the bottom of thecorresponding groove 20 of the disc 12 in order to immobilise the blade10 radially on the disc 12. “Leeks” 14 extending towards the inside ofthe fan are formed on the inside surface of the disc 12 in order tobalance the disc 12.

The disc 12 comprises a truncated cone-shaped wall 200 closing in thedownstream direction and extending from a portion of the disc 12 locatedradially between the grooves 20 and the “leeks” 14. The downstream endof the truncated cone-shaped wall 200 comprises a radial annular collar202 featuring axial orifices interacting with axial orifices of a radialannular collar 204 formed upstream from the drive shaft 13, for passageof bolts 206.

The inter-blade platforms form a wall that internally delimits a stream26 of the air flow entering the turbomachine and comprising means thatinteract with matching means provided on the disc 12, between thegrooves 20, in order to fix the platforms on the disc.

The fan blades 10 are retained axially in the grooves 20 of the disc 12via appropriate means mounted on the disc 12, upstream and downstreamfrom the blades 10.

The retaining means located upstream comprise an annular flange 28attached and fastened coaxially to the upstream end of the disc 12.

The flange 28 comprises an inner annular edge 30 which is scalloped orcastellated and interacts with a castellated or scalloped outer annularedge 32 of the disc 12 in order to axially immobilise the flange 28 onthe disc 12. This flange 28 is supported by an outer edge 34 on theshims 24 of the blade roots 18.

The flange 28 furthermore comprises an inner annular collar 36, which isinterposed between a corresponding annular collar 38 of the disc 12 andan inner annular collar 40 of a shell 42 arranged upstream from the fandisc 12. The collars 36, 38, 40 comprise axial orifices (not visible)through which screws 44 or similar pass for clamping the collars to oneanother.

The shell 42 has a substantially truncated cone shape flaring out in thedownstream direction, wherein the inter-blade platforms extend in theaxial extension of this shell 42. This shell comprises radial drillholes 46 for installing balancing screws in addition to a collar 48located at its upstream end. A conical cowling 50 is mounted on theupstream portion of the shell 42. More specifically, the downstream endof the cowling 50 comprises a collar 52, fixed to the upstream collar 48of the shell 42 by means of screws 54.

Downstream from the blade 10, a hook 120 formed at the downstream end ofthe blade 10 allows axial retention and engages in a notch 122 formed atthe upstream end of a compressor 124 prolonging the stream 26 downstreamfrom the fan.

Such a structure has the disadvantages described above. In particular,it is not suitable for a fan of relatively small dimensions.

FIGS. 3 and 4 illustrate an embodiment of a fan according to thesolution developed in this patent application and comprising, withregard to FIG. 3, a disc 56 bearing blades 132, the roots 138 of whichare engaged in substantially axial grooves 58 of the outer circumferenceof the disc 56 and in case of FIG. 4, a disc 56 formed in one piece withthe blades 132.

The disc 56 is arranged around the axis 130 of the turbomachine and isdriven in rotation by a downstream drive shaft 208.

More specifically, the disc 56 is connected to a truncated cone-shapedwall 210 extending downstream from the disc 56 and closing. Thedownstream end of the truncated cone-shaped wall 210 is connected to acylindrical wall 212, the inside surface of which comprises axialsplines 214 arranged circumferentially side by side. These splines 214,directly connected to the disc 56, are engaged by positive interlockingwith matching splines 216 arranged on the outer surface of the driveshaft 208.

The shaft 208 possesses, formed on its outer surface downstream from thesplines 214, 216, a first annular shoulder 218 interacting by axialabutment against the downstream end of the cylindrical wall 212connected to the disc 56 and bearing the splines 214. A second annularshoulder 220 formed upstream from the splines 214, 216 is in axialabutment against an annular edge 222 extending radially towards theinside from the truncated cone-shaped wall 210.

A nut 224 interacts with a thread 226 formed on the outer surface of theupstream end of the shaft 208 and rests axially in the downstreamdirection against the radial annular edge 222, so that the latter, inaddition to the downstream end of the cylindrical wall 212, cannotbecome disengaged from their abutments against the shoulders 218, 220 ofthe shaft 208. In this manner, the disc 56 is axially, radially andcircumferentially constrained in relation to the drive shaft 208.

This installation by splines between the disc 56 and the shaft 208 hasthe advantages of mechanical strength described above, particularly forsmall-sized fans.

In the specific case of FIG. 3, each fan blade 132 comprises a vane 136connected at its radially inner end to a root 138 which is engaged in asubstantially axial groove 58 of a shape matching the disc 56, formedbetween two ribs 140 of the disc 56 and allowing radial retention ofthis blade 132 on the disc 56.

The fan blades 132 are retained axially in the grooves 58 of the disc 56via the means 74, 86, 70, 96 described below with reference to FIGS. 5to 9 and arranged upstream from the blades 132.

A shim 142 is interposed between the root 138 of each blade 132 and thebottom of the corresponding groove 58 of the disc 56 in order toimmobilise the blade 132 radially on the disc 56.

Inter-blade platforms 134 are interposed circumferentially between theblades 132. The inter-blade platforms 134 form a wall that internallydelimits the stream 144 of the air flow entering the turbomachine andcomprising means that interact with matching means provided on the disc56, between the grooves 58, in order to fix the platforms on the disc.

The blades 132 are surrounded by an outer annular casing 146 delimitingthe air intake of the turbomachine. The outer casing 146 comprises aninner annular wall 148 externally delimiting the stream 144 of the airflow entering the turbomachine and in relation to which the outer endsof the blades 132 rotate circumferentially.

The hub ratio of the fan illustrated corresponds to the ratio of thedistance B between the axis 130 of the turbomachine and the inner limitof the stream 144 at the leading edge of the blade 132, divided by thedistance A between the axis 130 of the turbomachine and the outer endsof the blades 132. The fan illustrated has been designed such as toobtain a hub ratio that may be between 0.25 and 0.27, whereas thedistance A has a value of between 450 and 600 mm. This choice of hubratio involves using a disc, the outer limit of which, at the tops ofthe ribs, is at a distance C from the axis 130 of between 115 mm and 145mm.

Finally, the means 74, 86, 70, 96 of axial retention of the blades 132,which will be subsequently described, are sufficiently effective so thatunlike the fan of the prior art illustrated in FIGS. 1 and 2, the fanaccording to the invention illustrated in FIGS. 3 is devoid of hooks foraxial retention of the blades 132 arranged downstream from the blades132. On the contrary, as can be seen, the low-pressure compressor 150arranged downstream from the fan disc 56 abuts directly against thedownstream ends of the blade roots 138 and the ribs 140 of the disc.Therefore, there is no longer any radial depth constraint of the ribsrelated to engagement of the downstream hooks.

Consequently, the grooves 58 are radially shallower, with a depth D ofbetween 18 mm and 22 mm, than the grooves adapted for installation of ahook for axial retention of the blades. The shims 142 used to keep theblade roots 138 radially abutting against the ribs 140 are also radiallythinner. The ribs 140, thereby less elongated, are in this casesufficiently compact in order to resist deformations and breakage. Thisincrease in resistance of the ribs 140 allows construction of the discfrom a titanium alloy that is relatively light in comparison to aninconel alloy for example.

Furthermore, considering the new weight distribution of the discresulting from the change in height of the grooves, the inner wall ofthe disc 56 has been formed so as to have a balancing profile 152 of thedisc 56 that is different in relation to that of the prior art having“leeks”. This profile 152 of the wall is of truncated cone shape flaringout in the downstream direction and is formed by reaming. Proportionallyto the disc, this balancing profile 152 extends less than the “leeks”towards the inside of the turbomachine, up to a minimum radius Eincluded within the context of the invention between 60 mm and 70 mm,which represents the inner limit of the disc. Consequently, thisbalancing profile 152 is positioned radially on the outside of the nut224 for clamping the disc 56 to the drive shaft 208. This profile 152therefore allows passage of bulkier tools in the space for upstreamaxial access located around the axis 130 of the disc 56 and essentialfor installing the fan.

In the specific case of FIG. 4, the disc 56 is formed in one piece withthe blades 132, wherein the blades extend from the outer surface 57 ofthe disc 56. Hence, there is no need to form means of axial securing ofthe blades. Specific mounting of the disc 56 on the drive shaft 208using the nut 224 remains possible, as the balancing profile 152 can beformed in the same manner as in FIG. 3.

Reference will now be made to FIGS. 5 to 9, which illustrate morespecifically the means of axial retention of the blades, in the casedescribed with reference to FIG. 3. The disc comprises an annular rim 60devoid of balancing “leeks” and prolonged upstream by an annular portioncomprising an annular recess 62 delimited between an upstream face ofthe rim and a radial edge 64 extending towards the outside. The upstreamend of the annular portion comprises a collar 66 extending radiallytowards the inside at a distance from the edge 64 and comprising,regularly distributed over its entire circumference, axial holes 68through which screws 70, 72 pass. The edge 64 is scalloped orcastellated and comprises solid sections alternating with hollowsections.

The fan rotor is equipped with means of axial retention of the blades onthe disc in the upstream direction These means comprise a flange 74installed in the annular recess 62 of the disc 56 and forming an axialabutment of the blade roots.

The flange 74 comprises a substantially truncated cone-shaped wall 76flaring out in the downstream direction, the thickness of whichincreases in the downstream direction. The flange 74 is delimited at itsdownstream end by a radial face 78 abutting against the blades. Thedownstream end of the flange 74 comprises an inner annular edge 80 whichis scalloped or castellated and comprises solid sections alternatingwith hollow sections and has shapes substantially matching those of theedge 64 of the disc 56 to allow installation and removal of the flange74 in the annular recess 62 by axial translation, rotation of the flange74 in relation to the disc 56 and axial securing of the flange 74 in therecess 62 of the disc by abutment of the solid sections of the edge 80of the flange against the solid sections of the edge 64 of the disc.

The flange 74 finally comprises festoons 82 or hollow sections formed inalternation with solid sections 84 on its upstream edge.

The flange 74 is secured against rotation by means of a ring 86comprising a cylindrical section 88 delimited by inner and outercylindrical faces. The outer face comprises protrusions 90 extendingradially outwards and circumferentially along said outer surface of thecylindrical section 88 and inserted in the festoons 82 of the upstreamedge of the flange 74, providing an abutment against the solid sections84 of the upstream edge of the flange 74 to ensure locking againstrotation. The upstream edge of the ring is connected to ears 92extending radially inwards, formed with holes 94 through which screwspass. These ears are in upstream axial contact against the collar 66 ofthe disc 56 such that the holes 94 of the ears 92 are aligned with theholes 68 of the collar 66 and the cylindrical section 88 of the ring isin axial abutment from the outside against the collar 66 of the disc.The ring 86 may be executed in high alloy steel, in order to withstandbeing torn out.

The flange 74 is thus secured against rotation by abutment of its solidsections 84 against the protrusions 90 of the ring.

A cowling 96, made for example of aluminium and conical in shape, isfixed to the disc 12. For this purpose, the cowling 96 comprises, in itsmedian section, an internal annular edge 98 in which axial holes 100(through holes) are formed (FIG. 7), located opposite a hole 94 on twoof the rings 86 aligned with certain holes 68 in the collar 66 of thedisc 56. These holes 100 have screws 70 passing through, interactingwith nuts 102 mounted against the downstream section of the collar 66 ofthe disc 56 and allowing joining together of the cowling 96, the ringand the disc 56. The downstream section of the cowling 96 covers thering 86 and the flange 74 such that the internal stream 26 defined bythe inter-blade platforms extends in the axial prolongation of thedownstream section of the cowling 96.

As can be seen in FIG. 9, all the other holes 94 of the ring apart fromone, positioned opposite other holes 68 in the collar 66 of the disc 56have screws 72 passing through, interacting with nuts 104 and servingsolely to fix the ring 86 on the disc 56. The heads of these screws arehoused in blind holes 106 formed in the inner edge 98 of the cowling 96.

The inner edge 98 of the cowling 96 also comprises a cylindrical neckcollar 108 extending in the downstream direction, the end of which bearsagainst the inner end of the collar 66 of the disc.

The cowling 96 furthermore comprises radial threads 110 used to installbalancing screws, as is familiar from the prior art. In order toguarantee the correct position of these screws, the position of thecowling 96 needs to be indexed in relation to the fan rotor. For thispurpose, as illustrated in FIG. 8, an indexing pin 112 is installed inthe last hole 94 of the ring aligned with a hole 68 in the collar 66 ofthe disc 56. The pin 112 comprises a head 116 accommodated in a blindhole 114 of the inner edge 98 of the cowling 96, wherein the diameter ofthe head 116 of the pin 112 is determined such that it cannot beinserted in another blind hole 106, provided to accommodate the heads ofthe screws 72.

Reference will now be made to FIGS. 10 and 11, which represent the shims142, wherein the latter have been adapted to the reduction in depth ofthe grooves 58. Each shim more specifically adopts the form of atwo-sided plate 154, lying along the axis of the fan and placed againstthe bottom of one of the grooves 58. This shim is symmetrical in thethree axial, radial and circumferential directions, which avoids anyinstallation errors. Each side of the shim possesses its lateral edges156, or circumferential edges, which are chamfered, wherein the chamfers158 each form an angle of 10° with one side. The chamfers 158 of eachradially opposite side 154 join at the lateral ends of the shim, formingthe two lateral edges 156 of the shim. The junction angles between thesides 154 of the shim and the chamfers 158 are softened so as to exhibita curvature radius of between 1.50 mm and 1.80 mm and more specificallyof equal to 1.65 mm. The junction angles between the respective chamfers158 forming the lateral edges 156 of the shim are softened so as toexhibit a curvature radius of between 0.45 mm and 0.75 mm and morespecifically equal to 0.6 mm. Each shim 142 has a radial thickness ofbetween 1 mm and 3 mm, more specifically equal to 2 mm and a lateraldimension of between 17.0 mm and 18.2 mm and more specifically equal to17.6 mm.

The invention claimed is:
 1. A fan for a jet engine, the fan comprisingat an intake, fan blades, an annular casing, a hub adapted to rotatearound an axis and bearing the fan blades, each fan blade having aleading edge, and the fan blades extending radially relative to saidaxis in an annular stream delimited internally by the hub and externallyby the annular casing, wherein: said fan has an intake diameter, whichcorresponds to a diameter of a circle comprising radially outer ends ofthe fan blades, of a value of between 900 mm and 1200 mm and has a hubratio, which corresponds to a ratio of a diameter of an inner limit ofthe annular stream at radially inner ends of leading edges of the fanblades, divided by the intake diameter, of a value of between 0.20 and0.265; the hub comprises a fan disc having, at an outer circumference,ribs formed in alternation with grooves in which blade roots areengaged; a shim having a radial thickness of between 1 mm and 3 mm isinterposed radially between one of said blade roots and one of saidgroove bottoms; and said fan disc has an outer limit, formed by outerends of the ribs, a diameter of which is between 245 mm and 275 mm, andan inner limit, formed by an inner end of an inner wall of the fan disc,a diameter of which is between 120 mm and 140 mm.
 2. The fan accordingto claim 1, wherein the fan disc has an outer limit, formed by outerends of the ribs, a diameter of which is between 245 mm and 275 mm, andin that the grooves of the disc have a radial dimension between a bottomof the grooves and a top of the ribs, of a value between 18 mm and 22mm.
 3. The fan according to claim 1, wherein the internal wall of thefan disc is formed by an internal bore of a truncated cone shape,flaring out downstream, wherein an upstream end of the internal boredefines the inner limit of the fan disc, which inner limit defines aminimum radius of the fan disc.
 4. The fan according to claim 1, whereinthe fan disc bears between 17 and 21 fan blades.
 5. The fan according toclaim 4, wherein the disc bears between 18 and 20 fan blades.
 6. The fanaccording to claim 1, wherein the disc is made of titanium alloy.
 7. Thefan according to claim 6, wherein the titanium alloy corresponds to oneof TA6V, TU 7, and TA5CD4.
 8. A jet engine comprising the fan accordingto claim
 1. 9. A fan for a jet engine, comprising at an intake, fanblades, an annular casing, a hub rotating around an axis of theturbomachine and bearing the fan blades, which extend radially relativeto said axis in an annular stream delimited internally by the hub andexternally by the annular casing, wherein said fan has an intakediameter (A), which corresponds to the diameter of the circle comprisingthe radially outer ends of the fan Hades, of a value of between 900 mmand 1200 mm and has a hub ratio, which corresponds to a ratio of thediameter (B) of the inner limit of the stream at the radially inner endsof the leading edges of the fan blades, divided by the intake diameter,of a value of between 0.20 and 0.265, the hub comprising a fan dischaving, at an outer circumference, ribs formed in alternation withgrooves in which roots of the fan blades are engaged, the fan dischaving an outer limit, farmed by outer ends of the ribs, a diameter ofwhich is between 245 mm and 275 mm, and an inner limit, formed by aninner end of an internal wall of the disc, a diameter of which isbetween 120 mm and 140 mm.
 10. A jet engine, comprising: a fancomprising at an intake, fan blades, an annular casing, a hub rotatingaround an axis of the turbomachine, said hub comprising a fan discbearing the fan blades, which fan blades extend radially relative tosaid axis in an annular stream delimited internally by the hub andexternally by the annular casing, wherein: said fan has an intakediameter (A), which corresponds to the diameter of the circle comprisingthe radially outer ends of the fan blades, of a value of between 900 mmand 1200 mm and has a hub ratio, which corresponds to a ratio of thediameter (B) of the inner limit of the stream at the radially inner endsof the leading edges of the fan blades, divided by the intake diameter,of a value of between 0.20 and 0.265, and the hub comprises a fan disccomprising at an outer circumference ribs formed in alternation withgrooves in which blades roots of the fan blades are engaged, each groovehaving a groove bottom, and a shim having a radial thickness of between1 mm and 3 mm is interposed radially between one of said blade roots andone of said groove bottoms, and a low-pressure compressor arrangeddownstream from the fan disc and which abuts directly against downstreamends of the blade roots and the ribs of the fan disc, wherein said fandisc has an outer limit, formed by outer ends of the ribs, a diameter ofwhich is between 245 mm and 275 mm, and an inner limit formed by aninner end of an inner wall of the fan disc, a diameter of which isbetween 120 mm and 140 mm.
 11. The jet engine of claim 10, which isdevoid of hooks arranged downstream from the fan blades for axialretention of said fan blades, and the fan disc comprises: an annular rimprolonged upstream by an annular portion comprising an annular recess ofthe fan disc delimited between an upstream face of the annular rim and aradial edge of the fan disc extending externally, and a flange disposedin the annular recess and forming there an axial abutment of the bladeroots in the upstream direction.
 12. The jet engine of claim 11, whereinthe flange is delimited at a downstream end by a radial face abuttingagainst the fan blades, said downstream end of the flange comprising aninner annular edge which is castellated and comprises solid sectionsalternating with hollow sections and has shapes substantially matchingshapes of said radial edge of the fan disc, to allow an axial securingof the flange in the annular recess of the fan disc by abutment of solidsections of an edge of the flange against solid sections of the radialedge of the fan disc.
 13. The jet engine of claim 10, wherein thegrooves of the fan disc have a radial dimension between a bottom of thegrooves and a top of the ribs, of a value between 18 mm and 22 mm.